Miasma is a gaseous material generated by all demon kind. It saturates the air, ground and water and corrupts it from within. It has been known to turn dirt to ash, rot forest, and turn lakes and rivers to noxious slime.
Against living creatures, the miasma can cause insanity, physical and mental anguish. Prolonged exposure can even force sentient beings to become abominations possessing a rage and unyielding hunger that only death can release.
Paladins were essential in beating back the demonic armies, due to their latent immunity to the miasmic corruptive effects, and their ability to extend that immunity through their auras. Since the end of the War, there have been no other mortals born with the innate immunity.